Insights into the dynamic changes and chlorine reactivity of biomass-derived DOM after pyrolysis and oxygen-limited torrefaction of straw during freeze-thaw processes using fluorescence spectroscopy.

Large-scale production of biomass worldwide releases biomass-derived dissolved organic matter (BDOM) into water. Nevertheless, how different methods of processing biomass influence the characteristics and chlorine reactivity of BDOM in cold regions, especially under freeze-thaw (F/T) ageing remains unclear. Here, the spectroscopic characterization and specific formation potential of disinfection byproducts (SDBP-FPs) of BDOM released from straw after pyrolysis or oxygen-limited torrefaction, as well as their changing trends during F/T cycles were investigated. Optical analysis revealed different production methods distinctly affected the quantity and chemical and spectroscopic characteristics of BDOM. Notably, F/T ageing significantly affected properties and chlorine reactivity of BDOM. Different variation sequences and sensitivities of fluorescent components to F/T cycles in different BDOM types were attributed to chemical and molecular properties, and the 10-20th and 40th F/T cycles were essential which caused the clearest changes in the fluorophores. The variability in characterization and sensitivity between BDOMs resulted in significant differences in SDBP-FPs during F/T cycles, however, these differences were not associated with acute toxicity. Random forest models (RFMs) revealed that aromaticity, molecular weight and humification were driving factors for predicting chlorine reactivity. Moreover, various chemical properties and different fluorescent contents were strongly related to SDBP-FPs of BDOM during F/T ageing.